Wireless devices have been in use for many years for enabling mobile communication of voice and data. Such devices can include mobile phones and wireless enabled personal digital assistants (PDA's) for example. The transmission and reception of voice and data information by the wireless device is conducted with a base station, and in accordance with a particular standard, such as the Global System for Mobile Communications (GSM) standard.
The GSM standard dictates that wireless devices must transmit power at a specific power level. Therefore, the wireless device transmit circuits must ensure that the output power is constantly maintained at the specified power level, which is nominally 33 dB with a tolerance of plus or minus 1 dB for the GSM standard. Future standards may impose a constant output power level that is other than 33 dB. In the wireless device, a wireless transceiver chip includes both a receive core for receiving voice and data signals from the base station and a transmitter core for sending voice and data signals to the base station. One of the problems with maintaining a constant transmit power output level is that the circuits of the transmitter core will have performance that varies over temperature and process technology. For example, if the temperature of the wireless device exceeds a nominal operating temperature, then characteristics such as the output power can change. Semiconductor manufacturing process variation is a factor which can change the expected output power.
There are known techniques for correcting the output power level due to temperature and process variation effects. This includes both open loop and closed loop techniques. FIG. 1 is a block diagram showing a wireless device 10 having a closed loop power sensing scheme for regulating output power. Wireless device 10 includes a wireless transceiver 12 coupled to other discrete components used in the wireless input/output path, such as power amplifier 14, antenna switch 16 and antenna 18. The wireless transceiver 12 includes a transmitter core consisting of baseband to RF converter 20 and a variable gain amplifier 22, and a receive core 24. The baseband to RF converter 20 receives a digital signal from a base band processor, or microprocessor (not shown) of the wireless device 10, and executes well known signal processing operations to upconvert and prepare the signal for transmission. The variable gain amplifier 22 is set to provide a predetermined gain for the signal to be transmitted, usually with a gain control signal provided by the base band processor. The receive core 24 executes well known signal processing operations to downconvert and prepare the received signal for the base band processor.
The variable gain amplifier 22 is a circuit that is subject to process and temperature variation. For example, the output power of the variable gain amplifier 22 can shift by as much as 7 dBm, which is then further amplified by power amplifier 14. It is noted that the power amplifier 14 itself is subject to process and temperature variation. Therefore, to correct for these variations, a power corrector 26 is included for detecting the output power level of the power amplifier 14, and feeding back a correction signal to the power amplifier 14 for either increasing or decreasing the output power to meet the specified target level. This technique is sufficient for standards that require fixed output power levels, but then limits the wireless devices to that specific standard. Those skilled in the art will understand that other wireless communications standards will require that output power to be variable.
Typically, the base station in communication with the wireless device will instruct the wireless device to increase the gain for transmission, since the previously transmitted signals may have been detected as being sub-optimal. Those of skill in the art will understand that the request from the base station is embedded within the communication signal being transmitted to the wireless device. This increase can be specified as being a 10 dB increase, for example. Alternately, the base station can instruct the wireless device to reduce gain, in order to conserve battery power of the wireless device while maintaining optimal performance. Therefore, the closed loop power correcting system shown in FIG. 1 cannot be used for standards required variable power output.
One solution for correcting variable power output changes due to temperature is to set the gain in response to a sensed temperature. Most wireless devices usually include a temperature sensor for monitoring a temperature of the circuit board. Therefore, the final gain of the variable gain amplifier is characterized over different temperatures and the appropriate gain control signal is stored in memory. Table 1 is an example illustrating the type relational information that is stored in memory.
TABLE 1T1T2T3Code 1Code 2Code 3Gain 1Code 4Code 5Code 6Gain 2Code 7Code 8Code 9Gain 3
In response to a sensed temperature (T1, T2 or T3) during use and a desired gain (Gain 1, Gain 2, Gain 3) to provide the desired final output power, the appropriate code is obtained from memory and applied to the variable gain amplifier. As shown in Table 1, in order to obtain the gain level of “Gain 1”, different codes are used for sensed temperatures T1, T2 and T3. These temperatures can either be specific values or temperatures ranges for which the specific code is still valid. This is referred to as an open loop power correction system since the output power itself is not monitored in order to correct it. The problem with this system is that no accounting for process variation is provided because the characterized data is based on a nominal manufactured transceiver circuit. In order to account for process variation, each transceiver circuit should be characterized and have their own codes stored in memory. It should be apparent to persons skilled in the art that characterization of each wireless device requires significant amounts of time.
The wireless device 50 of FIG. 2 provides a closed loop solution for correcting variable power output. Wireless device 50 includes the same components shown in FIG. 1, except FIG. 2 replaces power corrector 26 with a power detector 52. Now, the output power is detected and sent to the base band processor. A characterization table can be included in memory of the base band processor for setting the appropriate gain in response to the sensed output power. The main problem with this solution is that the power detector is a discrete device whose characteristics will vary with temperature as well. Therefore, characterization of the power detector is required, and the data stored will be stored in memory with the power characterization data. Another problem is that the base band processor will require an auxiliary analog to digital converter for converting the detected output power into a digital signal.
Complicating the power output correction issue is the requirement for power ramping of the signal to be transmitted. Standards and governmental regulations in some areas require that the power level of the signal to be transmitted be ramped from a low power level to the maximum power level at the beginning of a transmission, and from the maximum power level to the low power level at the end of a transmission, as dictated by the particular standard. It is noted that the ideal ramping profile is different for different power levels. FIG. 3 illustrates an example power ramping profile of a signal between time t1 and t2. The ideal ramping curve 60 is to follow a raised cosine curve, and should be completed by time t2. Unfortunately, temperature can change the ramping profile such that ramping will either be too fast or too slow. A slow ramping of power results in insufficient data being sent to the base station as ramping has intruded into the data transmission time. Too fast and too much data is spuriously transmitted into the air, which may violate certain governmental regulations, or specifications set by one or more wireless standards. Therefore, additional ramping correction information must be stored in the base band processor.
Since full characterization of each assembled wireless device is too onerous a task, a single nominal wireless device is fully characterized to generate the appropriate codes for controlling the components of the transmitter core for correcting the output power and ramping profiles. Then subsequent devices have their output power tested and benchmarked against the nominal device. If there is variance in the output power relative to the nominal device, a compensation factor is applied to the codes of the table and then stored in memory. While this may reduce characterization time per wireless device, accuracy will be poor.
It is, therefore, desirable to provide a wireless device having a closed loop output power correction system with high accuracy while using minimal additional circuits and memory storage.